The present invention relates to a method and apparatus for detecting the presence of knock within an engine cylinder of an internal combustion engine.
It is important in the design of high performance internal combustion engines that improved fuel consumption and maximum power must be obtained with acceptable exhaust manifold temperatures and emissions control. In order to sustain the performance of the engine under these constraints, the engine must often be operated near the knock limit. Some engines in high performance automobiles have knock control which is a part of a microprocessor-based engine control system. In these systems the spark is advanced until knock is detected and is then retarded slightly to eliminate knock while still maintaining high engine efficiency. In order to do this it is necessary to know when a knock condition exists.
In these engines knock is detected by analyzing cylinder pressure pulses or acoustic vibrations. For example, a pressure transducer may be installed near the cylinder and connected through a gas path. When knock occurs, higher frequency fourier components of the pressure pulse are detected using a fast fourier transform device or an electronic filter. The principal drawback to this system is that it is too expensive to use on production automobiles and has been therefore relegated to use only as a laboratory tool.
Another method utilizes an accelerometer or microphone attached to the cylinder head. When knock occurs, mechanical vibrations in addition to those generated by normal combustion and engine operation are generated in the cylinder head and are picked up by the sensor. These vibrations are in the kilohertz range and are processed with a fast fourier transform device or an electronic filter. The signal is then used to help control the engine operation. Thus, the system is only used in relatively expensive automobiles; moreover, it is sensitive to additional extraneous engine noise which may be associated with valve operations or other mechanical events occurring within the engine. Very high performance engines with additional cylinders or valves per cylinder generate so much additional noise, however, that the transducer is frequently unable to discriminate between extraneous engine noise and knock.